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1. Field of the Invention
The present invention relates to lift boats that feature a hull having a plurality of legs, each leg having an associated jacking mechanism that enables the hull to be elevated or lowered relative to the legs and wherein each leg has a load bearing pad that engages the seabed during use. More particularly, the present invention relates to an improved lift boat having an improved leg, hull and pad configuration with improved stability, when jacked up and when underway, featuring port and starboard pads near the bow of the hull that extend laterally of the hull in respective port and starboard directions and that extend into recesses of the hull.
2. General Background of the Invention
Lift boats are well known in the art. These devices (sometimes called jack up barges or jack up rigs) include a floating hull that allows the boat to be transferred from one marine location to another. When the hull reaches a desired location, such as a proposed oil well or other job site, typically three or four legs are lowered from the hull or barge to the sea bed. These legs are then powered downwardly with jacking mechanisms to lift the hull vertically on the legs and above the water""s surface. Once in operating position, a jack-up rig presents a stable platform surface for oil and gas well drilling operations, work-over operations, repair or maintenance work etc., notwithstanding the wave action at the water surface below.
There have been many patents that have issued relating to jack-up rigs. An example of a recent patent that discloses a jack-up rig is U.S. Pat. No. 5,139,366 issued to Kenneth Choate and John Laird entitled xe2x80x9cOFFSHORE JACK-UP RIG LOCKING APPARATUS AND METHODxe2x80x9d. The Choate et al. patent provides a locking apparatus and method for an offshore jack-up rig having at least one leg extending through the hull and at least one set of rack teeth attached to each of the legs. One or more locking bars are supported from the hull and are movable in a direction substantially normal to the face of the rack teeth. A piston and cylinder power assembly moves the bars towards the teeth and a retention system engages the bars holding them in engagement with the teeth. The elevating system of the rig co-acts with the set bars to lock the hull and legs together.
Another recent patent that relates to jack-up rigs and explains there operation is U.S. Pat. No. 4,813,814 entitled xe2x80x9cLEG-HOLDING DEVICE FOR OFFSHORE PLATFORMxe2x80x9d.
Other examples of patents that have issued and relate generally to jack-up rigs include U.S. Pat. Nos. 4,722,640; 4,627,768; 4,589,799; 4,505,616; and 4,482,272.
A patent that illustrates the elevating and lowering of a jack-up rig in a marine environment is U.S. Pat. No. 5,224,798, entitled xe2x80x9cOVERLOADING DEVICE FOR A JACK-UP OIL PLATFORM AND PLATFORM INCLUDING THE DEVICExe2x80x9d (see FIGS. 4a-4f).
A common element of a lift boat is a lifting crane that can be used to lift supplies from its own deck, work boat, supply boat or the like, and place those supplies on the platform. Patents have issued that are directed to the placement of a crane on a jack up barge. Some years ago, a patented crane apparatus was designed to fit over the leg of a smaller sized lift boat wherein the leg was of a cylindrical pipe configuration. U.S. Pat. No. 4,417,664 disclosed generally the concept of mounting a crane having a gantry and a boom about the leg of a lift boat. Another patent that addressed the problem of mounting a crane on a jack-up rig (lift boat) where there is limited space is U.S. Pat. No. 4,652,177. This patent proposes to mount the crane on the jacking structure or jacking tower of the jack-up rig (lift boat).
The following U.S. Patents are incorporated herein by reference: U.S. Pat. Nos. 2,308,743; 3,183,676; 3,290,007; 3,367,119; 3,606,251; 3,750,210; 3,945,450; 3,967,457; 4,417,664; 4,456,404; 4,678,165; 4,813,814; 5,139,366; 5,580,189; 5,797,703; and all patents mentioned herein.
The apparatus of the present invention provides an improved lift boat having an improved configuration for its hull, legs, pads as well as placement of permanent deck loads such as the crane and deck house relative to the hull and pads (especially when underway).
What is provided is a lift boat having an improved configuration of hull, legs and pads, including three legs with relatively large pads on the legs which recess partially into the hull of the boat, and which extend partially outwardly and laterally when under way.
In the preferred embodiment, a portion of at least some of the pads extend laterally (eg. one pad to port and one pad to starboard) of the hull. The pads extend beyond the periphery of the hull, and can thus be much larger. This greater surface area of pads in contact with the sea floor lessens the likelihood there is that there will be a xe2x80x9cpunch throughxe2x80x9d of a leg into the sea floor, which would cause the boat to be unbalanced and possibly fall over. Additionally, these laterally extending pads supplement the aggregate buoyancy of the hull in normal underway operation.
There can optionally be included a sounding device in the bottom of each leg to assess the thickness of the crust of the sea floor. The sounding devices can be commercially available sonar devices which tie into the oscilloscope on the boat.
The purpose of the unique features of the present invention described herein is to improve the overall efficiency of the lift boat into which they are incorporated in several aspects. These features significantly increased load carrying capacity compared to conventional lift boats. Improved hydrodynamic performance is realized due to the shaping of the hull, the pads, and the beneficial combination of the combined shapes of the hull and forward pads with the pads in the retracted (or xe2x80x98upxe2x80x99) position.
The unique lift boat features of the present invention thus include oversized buoyant pads or structural footings attached to the bottom of each leg to support the increased payload weight the lift boat carries both in the hullborne (hull floating) and xe2x80x98jacked-upxe2x80x99 (legs in the xe2x80x98downxe2x80x99 position placing the pads on the sea-bottom with the lift boat suspended completely above the sea surface) modes.
By positioning the forward pads below the waterline in the xe2x80x98upxe2x80x99 or retracted position, the pads supplement the aggregate buoyancy of the boat in normal underway operation. An improved configuration or shaping of the forward part of the hull and the forward pads form a combined shape having reduced hydrodynamic drag, such that, though having a larger payload capacity, this lift boat satisfactorily operates with no more propulsion power than conventional lift boats. An improved, beneficial shaping of the hull above the forward pads increases its hydrostatic buoyant volume relative that of conventional lift boats for improved safety and stability.
The present invention is an improvement over the methods now being used in the prior art. The laterally extending and thus larger pads provide a larger footing on the sea floor so that down pressure on the sea bottom is reduced from that of current conventional lift boats for improved safety through reduced risk of sea bottom collapse under the pad contact pressure.
The larger forward pads are underwater in the normal xe2x80x98upxe2x80x99 position so that their buoyant volume is additive into the total buoyant volume of the boat. In the prior art, the normal practice is for such pads to be suspended above the waterline in the xe2x80x98upxe2x80x99 position).
The hull forward end, above and below the waterline, is uniquely shaped to increase its buoyant volume and to provide shaped recesses into which the pads retract in the xe2x80x98upxe2x80x99 position. The resulting aggregate or combined shape is designed for reduced drag compared to the conventional barge-like hull and irregularly immersed pads of a conventional loaded lift boat operating underway in wave conditions.